Silicon photonics modulator using TM mode and with a modified rib geometry

ABSTRACT

An optical modulator includes a rib; and a slab interconnected to both sides of the rib; wherein the rib is dimensioned relative to the slab to support guiding of a Transverse Magnetic (TM) mode with a main lobe that propagates orthogonal to the slab and with the main lobe substantially excluded from the slab. The rib guides wavelengths in an infrared range in the TM mode. A height of the rib, relative to the slab, is about half of a width of the rib, between the slab.

CROSS-REFERENCE TO RELATED APPLICATION

The present disclosure is a continuation of U.S. patent application Ser.No. 16/554,736, filed Aug. 29, 2019, and entitled “Silicon photonicsmodulator using TM mode and with a modified rib geometry,” which claimspriority to U.S. Provisional Patent Application No. 62/726,408, filedSep. 3, 2018, and entitled “Silicon photonics modulators using TM,” thecontents of each are incorporated by reference herein in their entirety.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to optical modulators. Moreparticularly, the present disclosure relates to a silicon photonicsmodulator using the Transverse Magnetic (TM) mode and/or with a modifiedrib geometry.

BACKGROUND OF THE DISCLOSURE

As known in the art, SiP utilizes silicon as an optical medium withsilicon patterned, with sub-μm precision, into photonic components. Theoperation is at wavelengths in the infrared range, usually around 1550nm which is used by most optical communication systems. Silicon-basedmodulators are widely used in optical communication systems. It is wellknown in the art that silicon modulators can be based on the use of a PNjunction in a rib waveguide. Such a waveguide is formed by providing athick silicon waveguide core region (200 to 250 nm thick, for example)surrounded by a thin slab (around 100 nm thick, for example). The PNjunction is typically formed laterally by doping one side of thewaveguide with an N-type dopant, and the other side with a P-typedopant. The P and N regions are electrically connected on respectivesides to electrodes.

In general, the design of a silicon-based modulator is a balancing oftradeoffs between phase efficiency, speed, and loss. A rib waveguide isa waveguide in which the guiding layer basically includes a slab with astrip (or several strips) superimposed onto it. Rib waveguides alsoprovide confinement of the wave in two dimensions. Existing approachesfor SiP modulators, including rib waveguides, do not meet therequirements for next-generation devices. For example, the requirementsfor next-generation devices include low Vπ, high bandwidth (BW), and lowOptical Losses (OL). Vertical and U-shaped PN junctions, for example,have good Vπ but have very low modulation BW (typically below 15 GHz).Also, the access resistance of such a structure is too high. Typically,most known solutions improve one figure-of-merit of the modulator (lowVπ, high bandwidth (BW), or low Optical Losses (OL)) at the expense ofthe others.

BRIEF SUMMARY OF THE DISCLOSURE

In an embodiment, a silicon photonics modulator includes a rib that is aPN junction; a slab including a P doped region adjacent to the waveguidecore on a first side and an N doped region adjacent to the waveguidecore on a second side, opposite the first side; and a first electrodeconnected to the P-doped region and a second electrode connected to theN-doped region, wherein the rib is dimensioned to support guiding of aTransverse Magnetic (TM) mode with a main lobe that propagatesorthogonal to the slab. The rib can guide wavelengths in an infraredrange in the TM mode. A thickness of the rib can be about half of awidth of the rib. A thickness of the rib can be about 400 nm, and awidth of the rib can be about 200 nm. A thickness of the rib can beabout four times a thickness of the slab. The P-dopant level and theN-dopant level can be each varied in the slab between the rib and thecorresponding electrodes. For the silicon photonics modulator, modulatorVπ can be about 2 V, optical losses can be about 4 dB, and bandwidth canbe around 26 GHz. A gap can be included in a middle of the slab wherethe rib is located, to shift an optical mode away from the slab. Anotherslab can be included on top of the waveguide to provide a tilted-Hwaveguide.

In another embodiment, a silicon photonics modulator includes a firstregion of a waveguide defined by a depletion width of a PN junction; asecond region of the waveguide defined by a remaining portion of thewaveguide from the first region; a third region that is a slab areasurrounding the waveguide and that includes a P-dopant on a first sideadjacent the waveguide and an N-dopant on a second side adjacent thewaveguide opposite the first side; and a fourth region surrounding thewaveguide, wherein the first region has a width similar to a width ofthe waveguide, supporting a Transverse Magnetic (TM) mode with a mainlobe that propagates orthogonal to the slab. The waveguide can guidewavelengths in an infrared range in the TM mode. A thickness of thewaveguide can be about half of a width of the waveguide. A thickness ofthe waveguide can be about 400 nm, and a width of the waveguide can beabout 200 nm. A thickness of the waveguide can be about four times athickness of the slab. The silicon photonics modulator can furtherinclude two electrodes each disposed on opposite sides of the thirdregion. The P-dopant level and the N-dopant level can be each varied inthe slab between the waveguide and the corresponding electrodes. For thesilicon photonics modulator, modulator Vπ can be about 2 V, opticallosses can be about 4 dB, and bandwidth can be around 26 GHz. A gap canbe included in a middle of the slab where the rib is located, to shiftan optical mode away from the slab. Another slab can be included on topof the waveguide to provide a tilted-H waveguide.

In a further embodiment, a method for providing a silicon photonicsmodulator includes providing a rib that is a PN junction; providing aslab including a P doped region adjacent to the waveguide core on afirst side and an N doped region adjacent to the waveguide core on asecond side, opposite the first side; and providing a first electrodeconnected to the P-doped region and a second electrode connected to theN-doped region, wherein the rib is dimensioned to support guiding of aTransverse Magnetic (TM) mode with a main lobe that propagatesorthogonal to the slab.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is illustrated and described herein withreference to the various drawings, in which like reference numbers areused to denote like system components/method steps, as appropriate, andin which:

FIGS. 1(a) and 1(b) are diagrams of a lateral PN junction basedmodulator with an applied reverse bias of a) 0 V and b) V_(max); FIG.1(c) is a diagram illustrating the various sections of the phaseshifters. The optical waveguide has a thickness of 220 nm, a width of500 nm and the slab thickness is 90 nm; and FIG. 1(d) is aPoynting-vector mode profile for the lateral PN junction based modulatorof FIGS. 1(a)-1(c).

FIGS. 2(a) and 2(b) are diagrams of a lateral PN junction basedmodulator with an applied reverse bias of a) 0 V and b) V_(max); FIG.2(c) illustrates the various sections of the phase shifters. The opticalwaveguide has a thickness of 400 nm, a width of 200 nm and the slabthickness is 90 nm, enabling use of the TM mode. FIG. 2(d) is aPoynting-vector mode profile for the lateral PN junction based modulatorof FIGS. 2(a)-2(c).

FIG. 3 is a diagram of the geometry of a TM-mode based silicon photonicsmodulator.

FIG. 4 is a diagram comparing the modulator of FIG. 1 that only guidesTE modes with the modulator of FIG. 2 that guides the TM mode.

FIG. 5(a) is a waveguide shape and FIG. 5(b) is a Poynting-vector modeprofile for a typical rib waveguide used for making an opticalmodulator.

FIG. 6(a) is a waveguide shape and FIG. 6(b) is a Poynting-vector modeprofile for a modified rib waveguide 70 according to the presentdisclosure.

FIG. 7 is an upside-down waveguide profile according to the presentdisclosure.

FIGS. 8(a) and 8(b) are diagrams of a tilted-H waveguide with h_(s)=45nm (total slab height of 90 nm) (FIG. 8(a)) and h_(s)=75 nm (total slabheight of 150 nm) (FIG. 8(b)). For both cases, h=220 nm and w=500 nm.

FIGS. 9(a) and 9(b) are Poynting-vector mode profiles for the modifiedtilted-H waveguide rib of FIGS. 8(a) and 8(b).

DETAILED DESCRIPTION OF THE DISCLOSURE

Typical silicon photonics (SiP) modulators are based on the use of PNjunctions in an optical waveguide. The waveguide is usually formed witha thicker silicon layer (typically 220 nm) surrounded by a thinner slabsection (typically thinner than 100 nm) as shown in a lateral PNjunction based modulator 10 in FIGS. 1(a) and 1(b). The shape of thefirst TE mode of the structure is also illustrated (lines 12). Mostmodulators 10 have a p-type dopant 14 on one side and an n-type dopant16 on the other side which creates a vertical intrinsic region 18. Thesestructures are called lateral PN junctions 20. The PN junctions 20 areconnected to two (metal) electrodes 22 positioned on each side of thewaveguide. The electrodes 22 are usually far to prevent excessiveoptical losses. When a reverse bias is applied to the electrodes 22, thedepletion width of the PN junction 20 increases, which changes theeffective index of the mode, as illustrated in the difference betweenFIGS. 1(a) and 1(b).

To simplify the following discussion, the optical modulator is splitinto four regions I, II, III, IV as shown in FIG. 1(c). Region I isdefined by the depletion width of the PN junction 20 biased at themaximum reverse bias voltage (V_(max)) allowed by the driver, applied tothe electrodes 22. The efficiency of the PN junction 20 (i.e., theV_(π)L_(π)) is solely defined by the characteristics of this section(dopant concentration, mode profile, etc.). Region II is the remainingsection of the un-etched optical waveguide. In this section, the opticalintensity of the mode is typically still very high. Region III is athird section which is the thin slab area surrounding the opticalwaveguide. This section connects the PN junction 20 to external circuitsvia the electrodes. The optical mode in this area is typicallyexponentially decaying. The combination of region II and III dominatesthe optical loss of the modulator 10 as well as the access resistance ofthe junction 20. From now on, these two sections are called an “accessregion.” Region IV is the SiO₂ surrounding the optical waveguide. The PNjunction 20 does not affect this portion of the optical signal.

Silicon modulators need to be designed carefully to optimize the varioustrade-offs of the structure. The doping concentration close to the peakof the optical mode (Region I) should be as high as possible to improvethe modulator efficiency and reduce its V. However, too high dopingconcentrations create modulators with a large junction capacitance,which leads to a lower modulation bandwidth. The access region (RegionII and III) should have a doping concentration as low as possible tominimize optical losses but, at the same time, should have a dopingconcentration as high as possible to minimize the access resistancehence improving the modulator bandwidth. A proper trade-off needs to bemade depending on the application.

The typical dimensions of the modulator 10, 500 nm×220 nm with a 90 nmrib, only guide the TE modes. Note, the first TE mode is guided, and atleast the second TE mode is guided as well. Both the size and thewavelength have an impact on the guided mode of the structure. The indexof refraction of the material is also critical to determine the guidedmode. An optical mode is completely defined by its mode profile and theeffective index.

Note, the various dimensions described herein are presented asapproximate values and variations are expected due to manufacturingtolerances, etc. Also, various descriptions with reference to themodulator 10, such as the Regions I, II, III, and IV, apply equally tothe silicon photonics modulator 30.

In various embodiments, the present disclosure relates to a siliconphotonics modulator using the Transverse Magnetic (TM) mode. The presentdisclosure proposes a different geometry, namely 200 nm×400 nm with a 90nm rib to enable the use of the TM modes. The silicon photonicsmodulator improves operation using the first TM mode instead of theusual Transverse Electric (TE) mode. Most SiP modulators are fabricatedon a 220 nm thin-film silicon layer. In this type of rib waveguides, TMmodes are not supported, which is why the TM mode is not used. Here, itis proposed to fabricate the modulator on a thicker silicon thin-filmthat allows the TM mode to be guided. The TM mode does not havesignificant optical intensity where there is a need to have a largedensity of carrier, namely Region II and III). As a result, the accessresistance of the PN junction can be reduced considerably providing alow Vπ modulator combined with a high bandwidth response. This siliconphotonics modulator is critical to meet the requirements for thetransmitter of a High-Bandwidth Coherent Optical Subassembly (HB-COSA).For example, this silicon photonics modulator can be used in differentform factors including CFP2-DCO, QSFP-DD, OSFP, etc. as well asvendor-specific modules.

In addition to presenting a silicon photonics modulator using the TMmode, the present disclosure includes new waveguide structures shown toimprove the mode confinement in the desired region. Specifically, byadding either a gap in the slab of a rib waveguide or a second slab, themode is more confined in the core and less in the slab. This modifiedrib geometry may be used with the silicon photonics modulator using theTM mode as well as other silicon photonics modulators such as using theTE mode.

For the next SiP modulator generation, there is a need to have amodulator with low V_(π), high bandwidth (BW) and low optical losses(OL). More exotic PN junctions then the usual lateral PN junction havebeen considered. Vertical PN junctions (the depletion region ishorizontal) and U-shaped PN junctions are well known in the literature.These high-efficiency PN junctions provide modulators with very low V.Unfortunately, the capacitance of such a structure is too high, and theaccess resistance cannot be reduced enough. As a result, such PNjunctions typically results in modulation BW lower than 15 GHz. If theBW needs to be increased, usually, the modulator length or the accessresistance of the modulator is reduced. Unfortunately, these schemesresult either in a degradation of the % and the OL figure-of-merits. Toconclude, most known solutions improve one figure-of-merit of themodulator at the expense of the others.

This silicon photonics modulators described herein are critical to meetthe requirement for the transmitter of the high-bandwidth COSA(HB-COSA). These applications need different form factors: CFP2-DCO,QSFP-DD, or OSFP for pluggable or, if not pluggable, then the HB-COSAwill sit inside a module.

As for today, the modulator requirements of the HB-COSA are as below:

1) The Vπ should be around 2.5 V (or lower);

2) The optical losses of the modulator should be around 5.7 dB (orlower); and

3) The modulation EO bandwidth should be 25-30 GHz (or higher).

For the baseline modulator available now, the Vπ is 3.5 V, the opticallosses are 5.7 dB, and the EO bandwidth is 20 GHz. This is far from thespecification for the HB-COSA. Starting from this baseline modulator,the first step that needs to be achieved is the reduction of themodulator Vπ to a value lower than 2.5 V since this specification isimportant. One possible way to achieve this goal is to increase themodulator length. Unfortunately, this will necessarily reduce themodulator BW to a value most likely close to 15 GHz. Afterward, the BWneeds to be improved by reducing the access resistance of the modulator.A BW improvement of about 5 GHz can be obtained without additionaloptical losses. However, the last 5 to 10 GHz will severely increase theoptical losses. The target specifications could then be obtained only byusing an external amplifier.

SiP Modulator Utilizing the TM Mode

The silicon photonics modulator proposed herein provides an improvementthrough a better designed optical waveguide. Ideally, one shouldmaximize the optical intensity located in Region I. In this situation,the phase shifts per unit of capacitance will be maximized as well.Furthermore, Region II and III should contain as little optical power aspossible. This will result in reducing the access resistance withoutincreasing the optical losses. These two points maximize the modulatorbandwidth.

To achieve this, it is proposed to use the TM mode of a rib waveguideinstead of the usual TE mode. As mentioned before, most SiP modulatorsare fabricated on a 220 nm thin-film silicon layer and the slab area istypically smaller than 100 nm. In this type of waveguide, TM modes arenot supported.

For conventional strip waveguides, both the TE and TM modes are guided.However, strip waveguides are not useful for optical modulators becausethere are no contacts to control them electrically. As a result, ribwaveguides, such as in FIGS. 1(a)-1(c), are used. If the rib is toothick compared to the core waveguide (˜220 nm), the TM mode stops beingguided. This is the case for the usual rib thickness (˜90 nm) used.

To allow the TM mode to be guided, the present disclosures increase thewaveguide thickness (˜400 nm) enough so that the TM mode is guided whileusing the usual rib thickness (˜90 nm). One could propose to keep theusual waveguide thickness constant (˜220 nm) and reduce the ribthickness instead (<<˜90 nm). This does work as well, and the TM mode isguided. This idea has the benefit of being directly compatible with theusual platform. However, most of the optical intensity is in thecladding (see FIG. 1(d)), which would make the modulator veryinefficient. This may be useful in some situations such as sensors, butlikely not for modulators. Of note, FIG. 2(d) illustrates the opticalintensity for the silicon photonics modulator 30 proposed herein.

FIGS. 2(a)-2(c) illustrate an implementation of the silicon photonicsmodulator 30 proposed herein such as using a 400 nm thick siliconwaveguide. The width is 200 nm, and the slab thickness is still 90 nm.Here, the charge concentration in the P and N region with the P dopant14 and the N dopant 26, respectively, is assumed to be the same as thosein FIG. 1. Since the depletion width of a PN junction (rib) 32 dependson the doping concentration, if the concentration of both the P dopant14 and the N dopant 16 are the same, the Region I width of both FIG. 1and FIG. 2 are going to be the same. FIG. 2 illustrates a lateral PNjunction 32. However, the idea can be implemented using any kind of PNjunctions.

The two optical waveguides in FIG. 1 and FIG. 2 have a similarrectangular dimension but the silicon photonics modulator 30 is rotatedby 90° relative to the modulator 10. That is, FIG. 2 is rotated 90°relative to the FIG. 1, while both are shown in a similar orientation.For example, the modulator 10 in FIG. 1 can have a thickness of 220 nm,a width of 500 nm and the slab thickness is 90 nm, and the modulator 30in FIG. 2 can have a thickness of 400 nm, a width of 200 nm and the slabthickness is 90 nm.

The modulator 10 has a wide but thin waveguide (as shown in FIGS.1(a)-1(c)), while the silicon photonics modulator 30 has a narrow buthigh waveguide (as shown in FIGS. 2(a)-2(c)) resulting in an opticalmode more suited for optical modulators (a rotation of 90 degree of theaspect ratio of the waveguide). FIG. 4 is a diagram comparing themodulator 10 of FIG. 1 that only guides TE modes with the modulator 30of FIG. 2 that guides the TM mode. In FIG. 4, it is shown that bothmodulators 10, 30 has similar main lobes 40 except that the modulator 30has its main lobe 40 rotated by 90 degrees relative to the modulator 10.That is, the modulator 10's main lobe 40 is parallel to the slab,whereas the modulator 30's main lobe 40 is approximately perpendicular(orthogonal) to the slab.

In FIG. 4, the modulator 30 has similar main lobes 40 as the modulator10 except that the TM mode is rotated by 90 degrees. That is, the mainlobes of the modulator 30 are perpendicular to a slab 50. Wings 52 ofthe modulator 30 are similar to the wings 52 of the modulator 10, againexcept rotated by 90 degrees.

For the modulator 10, it is very confined vertically but not so confinedhorizontally. The situation is reversed for the TM mode in the modulator30. An imaginary line 54 shows the direction where the mode is lessconfined. Of course, the presence of the slab 50 modifies the modeprofile a bit, but the idea was to change the relative position of theslab 50 compared to the lines 54. In the modulator 10, the line 54 iscolinear with the slab 50 which makes the mode confinement inside theslab 50 not as good compared to the TM mode in the modulator 30 onewhere the line 54 is orthogonal to the slab 50. Thus, the optical modein the modulator 30 does not naturally penetrate in the slab 50, unlikein the modulator 10.

This has a few significant benefits:

1) The optical mode is laterally more confined in Region I, which willimprove the modulator Vπ in the modulator 30 compared to the modulator10.

2) Since Region I has a width similar to the waveguide width, Region IIbecomes very narrow in the modulator 30. The fraction of optical powerlocated in this section being smaller, the optical losses will besmaller as well in the modulator 30 compared to the modulator 10.

Because of the boundary conditions of the electromagnetic fields, themode does not enter significantly into Region III in the modulator 30.As a result, larger charge concentrations can be brought closer to theoptical waveguide hence reducing the access resistance withoutincreasing the propagation losses. This will improve the modulator BW.

Table 1 below shows the fraction of the optical mode in the variousregions of the waveguides shown in FIGS. 1 and 2. The TM mode containsabout 50% more optical intensity than the TE mode in Region I.Furthermore, and more importantly, the optical power located in RegionII is reduced by more than 50%. This results in a significant reductionof the optical losses. Finally, the amount of optical power in RegionIII is also a bit less than two times lower for the TM mode compared tothe TE mode. As a result, the access resistance will be reducedsignificantly without modifying the overall optical losses.

One can also notice that there is a larger amount of optical mode in thecladding area for the TM mode compared to the TE mode. In principle, itis better to have a large portion of the mode intensity in Region I.However, it is also necessary to have as less optical intensity inRegion II and III as possible. As a result, if the optical intensitycannot be confined inside Region I, it is better to have it in Region IV(instead of Region II and III) to prevent the addition of excess losses.

TABLE 1 The fraction of the Poynting-vector profile for the waveguidesshown in FIGS. 1 and 2 Wave- Wave- guide guide Slab Portion of theoptical mode Polar- width height height in Region I to IV ization (nm)(nm) (nm) I II III IV TE 500 220 90 0.33 0.39 0.038 0.241 TM 200 400 900.55 0.13 0.0240.03 0.30

Implementing this silicon photonics modulator requires a silicon layerthicker than the usual 220 nm thin film. To get this thicker siliconlayer (300 nm, 400 nm, etc.), there are two possibilities. The firstapproach is to start with an initial wafer having the right thickness.However, components already designed for 220 nm Si film will either haveto be redesigned or, alternatively, the process could be modified suchthat an extra etching step would be added to bring some areas of thewafer to the desired 220 nm thick silicon film. The biggest challengewith this approach is to obtain blank Silicon on Insulator (SOI) wafershaving the proper characteristics (silicon film thickness, siliconresistivity, BOX thickness, etc.). The second approach could be tolocally grow poly-silicon or monocrystal silicon above the thin 220 nmfilm that will be used for the modulator 30. As a result, the modulator30 will have a thicker structure while leaving the rest of the waferunchanged.

The silicon photonics modulator 30 presented above can reach therequired specifications. To illustrate the gain obtained from thissilicon photonics modulator 30, the performances of the modulator 30shown in FIG. 3 has been simulated using the same simulation tools asfor the baseline modulator. The modulator Vπ is 2 V, the optical lossesare 4 dB, and the expected bandwidth is around 26 GHz. The modulator 30can further be optimized using the approaches described in PCT PatentApplication No. PCT/US2019/015258, filed Jan. 25, 2019, and entitled“Silicon-based modulator with optimized doping profiles and differenttransition zone thicknesses,” the contents of which are incorporatedherein by reference. As a result, using the TM mode is a good path toquickly reach the SiP modulator figure-of-merit required for the nextproduct generation.

In an embodiment, the silicon photonics modulator 30 includes a rib 32that is a PN junction; a slab 50 including a P doped region adjacent tothe waveguide core on a first side and an N doped region adjacent to thewaveguide core on a second side, opposite the first side; and a firstelectrode 22 connected to the P-doped region and a second electrode 22connected to the N-doped region, wherein the rib is dimensioned tosupport guiding of a Transverse Magnetic (TM) mode with a main lobe 40that propagates orthogonal to the slab 50.

The rib 32 guides wavelengths in an infrared range in the TM mode. Forexample, the wavelengths can be in the 1500 nm-1650 nm range. In anembodiment, the wavelengths can be in the 1525 nm-1565 nm range to coverthe C-band in fiber optic communication systems. A thickness of the rib32 can be about half of a width of the rib 32, where the thickness ishow much the rib 32 extends out from the slab 50. A thickness of the rib32 can be about 400 nm, and a width of the rib 32 can be about 200 nm. Athickness of the rib 32 can be about four times a thickness of the slab50.

The P-dopant level and the N-dopant level can be each varied in the slabbetween the rib and the corresponding electrodes. For the siliconphotonics modulator, modulator Vπ can be about 2 V, optical losses canbe about 4 dB, and bandwidth can be around 26 GHz. A gap 72 can beincluded in a middle of the slab 50 where the rib 32 is located, toshift an optical mode away from the slab 50. Also, another slab can beincluded on top of the waveguide to provide a tilted-H waveguide.

In another embodiment, a first region of a waveguide defined by adepletion width of a PN junction; a second region of the waveguidedefined by a remaining portion of the waveguide from the first region; athird region that is a slab area surrounding the waveguide and thatincludes a P-dopant on a first side adjacent the waveguide and anN-dopant on a second side adjacent the waveguide opposite the firstside; and a fourth region surrounding the waveguide, wherein the firstregion has a width similar to a width of the waveguide, supporting aTransverse Magnetic (TM) mode with a main lobe that propagatesorthogonal to the slab.

SiP Modulator with a Modified Rib Geometry

The doping concentration of the P-dopant 14 and the N-dopant 16 needs tobe high in region III in order to decrease the access resistance andobtain a fast modulation. However, the presence of a fraction of theoptical mode in this region causes optical loss. FIG. 5(a) is awaveguide shape 60 and FIG. 5(b) is a Poynting-vector mode profile for atypical rib waveguide used for making an optical modulator.

As seen in the first line of Table 1, although containing only 3.8%, theregion III significantly contributes to the modulator optical loss sincea large amount of dopant in that region is required to provide fastoptical modulation.

In the present disclosure, it is proposed to modify the rib geometry inregions I and II to decrease the fraction of the optical mode in regionIII allowing for decreasing the optical loss while keeping substantiallythe same modulation efficiency and speed.

Rib-Gap

FIG. 6(a) is a waveguide shape 70 and FIG. 6(b) is a Poynting-vectormode profile for a modified rib waveguide 70 according to the presentdisclosure.

The modified rib waveguide 70 includes a gap 72 introduced in the middleof the slab (extending region IV) that causes the optical mode to beshifted higher in the core (regions I and II) with a decreased fractionin the slab. In order to maintain similar mode confinement, the heightof the waveguide at the center is increased to correspond to the overallheight of the previous rib waveguide.

TABLE 2 The fraction of the Poynting-vector profile shown in FIG. 3contained in the four regions illustrated in FIG. 1c. Wave- Wave- guideguide central Slab Portion of the optical mode Polar- width heightheight in section I to IV ization (nm) (nm) (nm) I II III IV TE 500 22090 0.349 0.384 0.019 0.248

One sees that this new geometry allows decreasing by about a factor oftwo the modal fraction in region III. A significant decrease of themodulator optical loss is expected accordingly while the accessresistance remains virtually unchanged.

To fabricate such a new configuration, it might be easier to build thestructure upside down, as shown in FIG. 7. Noteworthy, a silicon layercould be deposited over an already patterned and flatten structurecomposed of a silicon waveguide surrounded by silica. Then the new layercan be patterned as well to introduce the desired gap as shown in FIG.7.

Flipping the structure as shown in FIG. 7 could also ease the definitionof side contact having a thinner thickness. Indeed, considering the ribwaveguide shown in FIG. 4, it is difficult to define a slab waveguide oneach side of the core having a desired very thin thickness since theaccuracy is most often corresponding to a fraction of the etchedmaterial. The etching accuracy is typically about +/−10%. Thus, defininga 50 nm slab starting from an initial thickness of 220 nm means that 170nm of silicon needs to be etched. Assuming a +/−10% accuracy in theetching depth, the resulting thickness would be 50+/−17 nm correspondingto +/−34%.

Using rather the structure shown in FIG. 7, a thin slab could be definedwith an improved accuracy.

The silicon layer deposited on top of an already patterned structure canbe of different nature. It can be mono or poly-crystalline. It can beintrinsic or doped. The deposited layer could also be of a materialdifferent from silicon, as long as it provides the connectivity(allowing for moving electrical charges) while not causing excessiveoptical loss. It could be, for example, a semiconductor other thansilicon.

Tilted-H Waveguide

It is possible to reduce even more the optical mode overlap into theslab region by combining the previously proposed top silicon layertogether with a conventional rib waveguide to create a titled-H shapedwaveguide 80 as shown in FIGS. 8(a) and 8(b). This new tilted-Hwaveguide 80 has two times more access waveguides. The correspondingPoynting-vector mode profiles are shown in FIGS. 9(a) and 9(b) and theproportion of the Poynting vector in each of the four regions areprovided in Table 3 as well as the proportion of more conventional ribwaveguide (for comparison purpose). Each access waveguide has a height(h_(s)) and to easily compare the proportion of power inside the varioussections of the waveguide discussed in FIG. 1, now region III containsthe four slabs, and the total rib height is equal to 2h_(s). Thewaveguide width (w) and height (h) are defined identically then regularrib waveguides.

TABLE 3 The fraction of the Poynting-vector profile for rib waveguideswith slab height of 90 nm and 150 nm respectively and for tilted-Hwaveguides with a total slab height of 90 nm and 150 nm Wave- Wave-guide Total guide central slab Portion of the optical mode Polar- Geom-width height height in section I to IV ization etry (nm) (nm) (nm) I IIIII IV TE Rib 500 220 90 0.334 0.387 0.038 0.241 TE Rib 500 220 1500.24  0.381 0.157 0.222 TE Tilted- 500 220 90 0.402 0.413 0.013 0.172 HTE Tilted- 500 220 150 0.381 0.436 0.031 0.152 H

As can be seen from Table 3, the tilted-H waveguide with a total slab of90 nm (i.e., h_(s)=45 nm) contains significantly less power in the slabregion (3.8% versus 1.3%). This can be explained by the fact that havinga slab both at the top and at the bottom of the waveguide attract theoptical mode toward the center which reduces the overlap of the modeinside the slabs. Since the mode is more located around the center, theeffective index contrast is larger, which also increase the confinementof the mode in the core. Since the proportion of the optical mode insidethe core (region I+region II) is more localized inside the depletionregion (region I) (i.e., the confinement of the mode inside the activeregion can be defined by

$ {\Gamma = {\frac{P_{dep}}{P_{core}} = \frac{P_{I}}{P_{I} + P_{2}}}} ),$

the benefit on the V, (region I) is going to be slightly dominating thedegradation due to the optical loss (region II), which is also a sidebenefit of this type of waveguide with h_(s)=45 nm.

It is also of interest to reduce the access resistance in addition tooptical losses (or instead of optical loss). As a result, one canincrease the doping concentration inside region III. However, thetrade-off between optical losses and access resistance becomes lessadvantageous as the bandwidth and/or the modulation efficiencyrequirements increases. Another approach is to increase the slab height.However, as can be seen from the first two lines of Table 3, increasingthe slab height of typical rib waveguide is significantly detrimental toboth the confinement in the region I (which reduces the Vπ) and inregion III (which increases the optical losses).

Interestingly, one important benefit of the tilted-H waveguides is thatan increase in the total slab height (to the value of 150 nm) is notstrongly affecting the mode confinements compared to usual rib waveguidewith a slab of 90 nm (first line of Table 3). Both region I and regionII contain more optical power both in a similar proportion (F remainsconstant). Thus the relationship between the modulator V, and itsoptical loss will be similar. So, modulator with identical V, can beobtained from PN junction of line 1 and 4 from Table 3 with similaroptical losses, but modulator with PN junctions of line 4 will beshorter (about 15% shorter) which is beneficial for the bandwidth.Furthermore, the fraction of power inside the slabs are similar (3.1%versus 3.8%) again with a small advantage for line 4. However, since theoverall thickness of the slab of line 4 is 5/3 larger, the accessresistance (for identical dopant concentration) will be reduced by thisfactor. All of this will provide a significant benefit for the modulatorbandwidth.

It should finally be noted that other combinations or variations of thesame ideas are of interest, for example:

Providing a top slab that includes a gap and serves at carrying theelectric charges combined with a bottom slab without a gap that ispassive and serves only at delocalizing the mode still lower;

Providing top and bottom slabs that both include gaps and serve atcarrying the electric charges; and

The tilted-H waveguides containing a gap in the top slab.

Also, the TM modulator 30 can be combined with the tilted-H waveguide orto the top-rib waveguide ideas.

Although the present disclosure has been illustrated and describedherein with reference to preferred embodiments and specific examplesthereof, it will be readily apparent to those of ordinary skill in theart that other embodiments and examples may perform similar functionsand/or achieve like results. All such equivalent embodiments andexamples are within the spirit and scope of the present disclosure, arecontemplated thereby, and are intended to be covered by the followingclaims

What is claimed is:
 1. An optical modulator comprising: a rib; and aslab interconnected to both sides of the rib; wherein the rib isdimensioned relative to the slab to support guiding of a TransverseMagnetic (TM) mode with a main lobe that propagates orthogonal to theslab and with the main lobe substantially excluded from the slab.
 2. Theoptical modulator of claim 1, wherein the rib guides wavelengths in aninfrared range in the TM mode.
 3. The optical modulator of claim 1,wherein a height of the rib, relative to the slab, is about half of awidth of the rib, between the slab.
 4. The optical modulator of claim 1,wherein a height of the rib, relative to the slab, is about 400 nm, anda width of the rib is about 200 nm, between the slab.
 5. The opticalmodulator of claim 1, wherein a height of the rib, relative to the slab,is about four times a height of the slab.
 6. The optical modulator ofclaim 1, wherein the slab includes variable doping.
 7. The opticalmodulator of claim 1, wherein the rib includes two regions, Region I andRegion II, with the TM mode confined in the Region I.
 8. The opticalmodulator of claim 7, wherein the Region II is smaller than the Region Ireducing optical losses.
 9. The optical modulator of claim 7, whereinthe slab includes a Region III where, because of the boundary conditionsof the electromagnetic fields, the main lobe is substantially excludedfrom the slab.
 10. The optical modulator of claim 1, wherein the opticalmodulator is a silicon photonics device.
 11. The optical modulator ofclaim 1, wherein the rib a PN junction, and the slab includes a P dopedregion adjacent to the rib on a first side and an N doped regionadjacent to the rib on a second side, opposite the first side.
 12. Theoptical modulator of claim 11, further comprising a first electrodeconnected to the P-doped region and a second electrode connected to theN-doped region.
 13. An optical modulator comprising: a first region of awaveguide defined by a depletion width of a PN junction; a second regionof the waveguide defined by a remaining portion of the waveguide fromthe first region; a third region that is a slab area surrounding thewaveguide and that includes a P-dopant on a first side adjacent thewaveguide and an N-dopant on a second side adjacent the waveguideopposite the first side; and a fourth region surrounding the waveguide,wherein the first region has a width similar to a width of thewaveguide, supporting a Transverse Magnetic (TM) mode with a main lobethat propagates orthogonal to the slab.
 14. The optical modulator ofclaim 13, wherein the waveguide guides wavelengths in an infrared rangein the TM mode.
 15. The optical modulator of claim 13, wherein a heightof the waveguide, relative to the slab area, is about half of a width ofthe waveguide, between the slab area.
 16. The optical modulator of claim13, wherein a height of the waveguide, relative to the slab area, isabout four times a height of the slab area.
 17. The optical modulator ofclaim 13, wherein the second region is smaller than the first regionreducing optical losses.
 18. The optical modulator of claim 13, whereinthe third region, because of the boundary conditions of theelectromagnetic fields, has the main lobe is substantially excludedtherefrom.
 19. The optical modulator of claim 13, wherein the opticalmodulator is a silicon photonics device.
 20. The optical modulator ofclaim 13, further comprising a first electrode connected to first sideand a second electrode connected to the second side.